In the gypsum drywall industry taping compounds are used, some of which must be mixed with potable water prior to application. In the plastering industry, pails of premixed plaster weighing in the order of 25 kg must be mixed with cement before being applied as a patching and finishing compound. In both industries, a portable mixer having mixing paddles which rotate at approximately 400-600 RPM is used to mix the cementatious or viscous materials. When the mixer is inserted into the cementatious or viscous materials in the pail, the pail tends to rotate with the mixer. To prevent this the installer must use his or her legs, feet or other means to rigidly hold the pail so as to prevent the pail from turning as the material is mixed. This is potentially dangerous, as mixing in this manner may cause injury or loss of balance to the worker because of the awkward position which he or she must assume to maintain control over the operation.
Various techniques and designs have been previously suggested for stacking and limiting movement of pails and similar containers.
U.S. Pat. No. 4,311,238, Smith, issued Jan. 19, 1982, teaches containers 11 and 11' having bottom rims 16 and 16', respectively. Containers 11 and 11' may be stably stacked one on top of another by using closures 12. Closures 12 seal containers 11 and 11'. Closures 12 have lugs 41 which vertically and laterally support bottom rims 16 and 16'. Lugs 41 are circumferentially spaced protrusions, protruding inwardly from annular wall 33. Lugs 41 have upwardly opening, generally concavely curved surface segments 46 which substantially conform to the upwardly directed generally convexly contoured inner portion of bottom rims 16 and 16'. Bottom rims 16 and 16' are circumferentially continuous around containers 11 and 11'.
Smith does not teach preventing rotation of containers 11 and 11' when stacked on closures 12. Bottom rims 16 and 16' are free to rotate on surface segments 46. Notching of bottom rims 16 and 16' so as to engage lugs 41 is not taught due to the paperboard construction of containers 11 and 11' which requires that bottom rims 16 and 16' be left circumferentially continuous to retain the structural integrity of containers 11 and 11'.
U.S. Pat. No. 3,079,037, Schechter, issued Feb. 26, 1963, discloses, in one embodiment, open topped rectangular receptable 29 and tray-closure element 28. Tray-closure element 28 fits into the open top of receptable 29 so as to close receptacle 29. Tray-closure element 28 also fits onto the bottom of receptacle 29, and is held there by the engagement of bead 39 on receptacle 29 with groove 41 on tray-closure element 28.
Schechter does not suggest using tray-closure element 28 to prevent rotation of receptacle 29 relative to tray-closure element 28, although that is the effect of the rectangular embodiment of the container with tray-closure element 28 attached to its bottom surface. A rectangular embodiment is not, however, practically useful as a dry wall plaster pail, because mixing cannot be efficiently effected in the corners of the rectangular container. Thus, rectangular containers cause waste, as the unmixed plaster is unusable, and further introduce the chance of inferior dry walling, in that unmixed plaster might be inadvertently employed along with mixed plaster. The round embodiments of the container and tray-closure element do not act to prevent rotation of the container relative to the tray-closure element when the tray-closure element is attached to the bottom of the container because bead 16 is free to rotate within groove 27.